1. Field of the Invention
The present invention relates to optical units, exposure apparatus, and device manufacturing methods, and more particularly to an optical unit that has an optical path inside its barrel, an exposure apparatus that comprises the optical unit, and a device manufacturing method that uses the exposure apparatus for manufacturing electronic devices.
2. Description of the Related Art
Conventionally, various exposure apparatus have been used in a lithographic process for producing devices such as a semiconductor device and a liquid crystal display device. Among these apparatus, the ones mainly used in recent years are a reduction projection exposure apparatus (a so-called stepper) based on a step-and-repeat method, which reduces and transfers a pattern formed on a mask (also referred to as a reticle) proportionally enlarged around four to five times onto a substrate subject to exposure such as a wafer via a projection optical system, and a scanning projection exposure apparatus (a so-called scanning stepper) based on a step-and-scan method, which is an improvement of the stepper, or the like.
With these projection exposure apparatus, exposure wavelengths have shifted to a shorter range in order to achieve high resolution corresponding with finer integrated circuits. At present, a KrF excimer laser having a wavelength of 248 nm is mainly used, however, an ArF excimer laser having a shorter wavelength of 193 nm is gradually beginning to be used in practice. Furthermore, proposals are recently being made on projection exposure apparatus using an F2 laser that has a shorter wavelength of 157 nm, and an Ar2 laser that has a wavelength of 157 nm.
Such beams in the wavelength range called vacuum ultraviolet are strongly absorbed by almost all substances. For example, oxygen, water vapor, and almost all organic substances strongly absorb light in this region. Accordingly, in an exposure apparatus that uses vacuum ultraviolet light as its light source, the gas along the space of the optical path where the exposure light passes through needs to be replaced with a specific gas having properties of absorbing only a small amount of the vacuum ultraviolet light (hereinafter referred to as “low absorptive gas”), such as, for example with a rare gas like nitrogen or helium, so that the concentration of the absorptive gas such as the above oxygen along the space of the optical path is suppressed below several ppm.
Even when ultraviolet light which wavelength is slightly longer than that of the vacuum ultraviolet light is used (wavelength around 193 nm), oxygen is preferably removed from the optical path since the ultraviolet light will be absorbed by the oxygen.
The inside of the projection optical system should be filled with a rare gas such as nitrogen or helium, as is described above. However, helium gas is extremely effective for suppressing an increase in temperature which occurs when lenses or the like that are constituent elements of an optical system such as the projection optical system absorb the exposure light, that is, from the viewpoint of cooling effect.
The constituent elements of the projection optical system, such as the lenses, are housed in a barrel.
The refractive index of helium gas (around 1.000038), however, greatly differs from the normal refractive index of oxygen or nitrogen (around 1.000319). Therefore, when gases such as nitrogen or oxygen enter the barrel filled with helium gas from the periphery, the refractive index of the gas inside the barrel greatly changes, which greatly deteriorates image forming characteristics of the projection optical system.
In general, it is fairly easy to secure air-tightness in a barrel since it is configured of solid metal members. On both ends of the optical path on the reticle side and the wafer side, however, lenses that require an extremely high surface accuracy are arranged. So, it is not appropriate to strongly fasten and hold the lenses to increase the air-tightness, because when this happens the lenses deform greatly due to stress, which leads to a decrease in the surface accuracy.
On the other hand, an image forming optical path, which is space from the reticle through the projection optical system (projection lens) to the wafer, may be entirely filled with helium gas. When the reticle or the wafer is exchanged, however, a huge amount of helium gas is consumed (leaks outside), creating problems from the viewpoint of running cost of the exposure apparatus. In addition, the gas filled within the barrel containing absorptive gas needs to be controlled so that the concentration of the absorptive gas is lower than that of gas in the periphery of the barrel. This is because when the gas in the periphery of the barrel enters the barrel, the absorptive gas absorbs the exposure light.